A. Field of the Invention
This invention relates to glass optical fibers which are defined as glass fibers having a radial variation of index of refraction confining the light to propagate along its length without substantially reaching its cylindrical surface and having an attenuation of less than 1 dB/km or 1000 dB/km.
This invention further relates to glass optical fibers having a surface layer different in composition from the interior which are drawn from preforms having a surface layer different in composition from the interior and such that in the fibers and preforms, the surface layer is in compression and the interior in tension. In particular this invention relates to a method of increasing the abraded strength of glass fiber waveguides.
B. Brief Description of the Prior Art
Fibers may be manufactured by a variety of techniques which exhibit in the pristine state tensile strengths of approximately 10.sup.6 psi. Mechanical and chemical damage to the surface aided by stress corrosion will quickly degrade the tensile strength of an unprotected fiber to tensile strengths on the order of 15,000 psi or less, reducing or eliminating the practical uses of such a fiber. The method of preserving fiber strength has been to apply a coating material such as a polymer to protect the fiber mechanically against damage due to abrasion and due to chemical corrosion form water or other causes. No protective coating will provide absolute protection against mechanical damage and chemical corrosion, nor can a coating prevent the stress corrosion caused by any mechanical stress on the fiber.
The guidance of those practiced in the art of producing glass clad optical fibers has been to carefully select compositions for the core and clad of such a fiber to match certain physical properties. "For each system it is necessary to vary the composition as between core and cladding to achieve the required refractive index difference, which must be small and well controlled. This usually results in differences in softening temperature and melt viscosity, and thermal expansion coefficient. As a result the final product is likely to have built-in stresses, and it is desirable that these should be reduced to a minimum to achieve optimum optical and mechanical properties.".sup.1 Others who are practiced in the art of producing optical fibers have recommended matching the coefficient of thermal expansion and the glass transition temperature of the core and cladding regions..sup.2,3 FNT 1. Foord, S. G., et al., "Some Design Principles for Fibre Optical Cables," Proc. 23rd Int'l. Wire and Cable Conf., Atlantic City, NJ, Dec. 3-5, 1974, pp 276-280. FNT 2. Pearson, D.C., et al., "Light Guidance in Glass Media," Amer. Ceram. Soc. Bul. 49, 969 (1970). FNT 3. Newns, U.S. Pat. No. 3,957,342 issued to Newns, et al., May 18, 1976.
It is well known to those practiced in the art or producing strengthened glass articles that causing the article to be manufactured with a surface layer in compression will prevent strength degradation due to chemical and stress corrosion.